Apple was absolutely an industry leader before the 2000s. Which leaders exactly have been knocked off their perches?
If you're talking about the current AT&T, then I'd argue it's not a great example. The relationship between old AT&T and the company currently calling itself AT&T is tenuous. It would be more accurate to say the brand name survived, because it has good name recognition value.
The comment talks about "quickly changes the world around us rather fundamentally", "knocked off their perch", and "within a few years", which doesn't seem like an accurate description of what happened with Yahoo.